Heat-sensitive recording papers mean those in which images are formed utilizing the physical and/or chemical change(s) in the materials induced by heat energy. A large number of processes relating to such papers have been investigated.
Recently, heat-sensitive recording papers have been employed for the output of facsimiles, and the output or recording paper of electronic computers because these papers are characterized by being based on primary coloration not requiring developing process and so on. These recording papers are called "dye-type papers" and are disclosed in Japanese Patent Publication Nos. 4160/68 and 1403/70 (corresponding to U.S. Pat. Nos. 2,663,654 and 2,967,785), and Japanese Patent Application (OPI) No. 27253/80 (corresponding to U.S. Pat. No. 4,283,458) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc.
In general, there exists an advantage that the recording devices can be lighten and be smaller size, when a heat-sensitive recording paper is employed as a recording paper. Accordingly, this recording paper has come to be widely used. On the other hand, heat-sensitive recording paper is not desirable because the recorded image obtained fades away due to the influence of the external conditions, i.e., humidity, heat, etc. The fading of the recorded image is a serious defect during practical applications and there are various proposals for the improvement of this point.
There exist some descriptions relating to the addition of a phenol derivative such as 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), etc., described in Japanese Patent Publication No. 43386/76 (corresponding to U.S. Pat. No. 3,937,864), the addition of a water-insoluble modified phenol resin such as one modified by rosin described in Japanese Patent Application (OPI) No. 17347/78, the addition of a terephthalic acid ester such as diethyl terephthalate described in Japanese Patent Application (OPI) No. 72996/81 and so on. However, all these methods are insufficient in the prevention of the fading and furthermore possess a defect that undesired coloring so called "fog" appeared in the process of the recording paper production or during the preservation of the recording paper. This fog can be substantially increased especially under the influence of high humidity and heat, and thus commercial value decreases greatly under such conditions.